1. Field of the Invention
The present invention relates to a charged particle irradiation method used for observation of a sample and a method for manufacturing a semiconductor device.
2. Description of the Related Art
In general, an electron microscope has been used as an apparatus for observing semiconductor patterns. In particular, in applications where patterns are observed and their dimensions/shapes are measured during the manufacturing process, a CD-SEM (Critical Dimension-Scanning Electron Microscope) has bee used widely which has an automatic measuring function built in.
As the dimensions of semiconductor devices have been scaled down, the diameter of an electro beam has been reduced and hence the resolution of the CD-SEM has been increased. Furthermore, as the dimension measuring function and the focus adjusting function have advanced, the measurement accuracy of the CD-SEM has greatly improved. For this reason, the CD-SEM has become one important tool in observing local pattern shapes and variations in dimension.
Semiconductor patterns include various types of patterns. One of the types of patterns which is important in improving the performance of semiconductor devices and the manufacturing yield is hole patterns. The hole patterns include contact hole patterns for connection to an Si substrate and via hole patterns for connection between interconnect lines. With these hole patterns, it is important that the dimensions and shapes of holes be proper.
In recent years, as the dimensions of semiconductor devices have been scaled down, the diameter of holes has decreased and their depth have increased; thus, the ratio of the hole depth to the hole diameter, that is, the aspect ratio, has greatly increased. For the formation of hole patterns having high aspect ratio, difficult processing conditions are selected. Therefore, highly accurate measurement and control is demanded to determine whether proper hole patterns have been formed.
Until now, it has been difficult to observe a hole pattern having high aspect ratio on a CD-SEM. The reason is that secondary electrons emitted from the bottom of a hole collide with the sidewall of that hole, failing to arrive at a secondary electron detector in the electron optical system.
As a method of observing the hole bottom brightly, a method has bee proposed which, immediately before observing a hole pattern at a high magnifying power, irradiates the hole pattern with an electron beam at a magnifying power lower than the observation power (Y. Ose, M. Ezumi, H. Todokoro, “Improved CD-SEM Optics with Retarding and Boosting Electric Field”, Proc. SPIE 3677, pp. 930-939 (1999)).
Under conditions of a high magnifying power, the area of an irradiation region is set to, say, 1.0 μm□. Under conditions of a low magnifying power, on the other hand, the area is set to, say, 100 μm□. The aforementioned method utilizes a phenomenon in which, by irradiating a wide region with an electron beam, the surface of a sample in this wide region is more positively charged than in a narrow region (hole pattern). This causes an effect of bringing secondary electrons at the hole bottom to the detector side. The aforementioned method has made it possible to observe the shape of the hole bottom and measure its dimensions.
However, no application of the aforementioned method to observation of a semiconductor device having various fine hole patterns has been reported and its effectiveness has not become apparent.